A traditional clutch generally transmits the rotational torque by combining two (or more) broad frictional surfaces and cuts the transmission of the torque by separating frictional surfaces, or permits the rotation by separating them and cuts the rotation by combining them. This type clutch needs broad frictional surfaces to transmit the torque and also needs a control device, and a hydraulic device to control the operation. There exists a possibility for damages and a segregation of components installed in the input side from the desired position by the back driving occurred in the wake of an unexpected reverse torque input from the output side.
A mechanical device, called a one-way clutch, which has two type (a roller, a sprag), exists to solve these reverse input problems by self-controlling either idling or the transmission of the rotational torque according to the rotational direction of the input shaft. It does not need a control device, like a hydraulic device, and makes each component driving in one way, which prevents the reverse directional rotation of each component. But when the unexpected torque from the output side according to the construction of the one-way clutch happens to occur, there still exists a high probability of damaging or segregating the location of equipment installed in the prefixed location in the input side.
A device to solve such a problem as back driving is proposed by lowering the general gearing efficiency. The goal is achieved by the negative outcome like using more driving power but reducing the overall system efficiency. For example, the worm and the worm gear are protected from the back driving when the torque is input from the output side of the gear device. Because this device is based on the over-running clutch drive structure which has a taper shape attached to the shaft, when the movement of the shaft has both the specific axial and angular direction, the gearing to the axial direction to prevent the back drive is achieved. This structure consists of many small components which can bring more expense. We need a new clutch assembly with a little change to the shaft; a new clutch which has fewer components than the present assembly and has a chooseable function to fix the system driving train (the prevention of the back drive) and optimizes the efficiency of the worm and the worm gear system to decrease the problem of the back driving at a higher level.
A sprag type reverse locking clutch has been to prevent the back driving without decreasing the overall system efficiency. The torque input from the input shaft is transmitted to the output shaft by making the sprag not contact to the housing but the torque input from the output shaft is not transmitted to the input shaft by making the sprag contact to the housing, which locks the output shaft. It does not need any control device to operate and is divided into the bearing part, in which a ball is installed, and the clutch part, in which a sprag is installed. The U.S. Pat. Nos. 6,695,118 and 6,871,735 also has a clutch assembly using the roller to lock the output shaft or using the roller to cut the transmission of the torque to prevent the back driving, respectively. They set up pockets between the input shaft and the output shaft. It works in a manner that rollers are inserted and engaged in the pockets. When the axis of the input shaft relatively mismatches to the axis of the output shaft due to the separation of the clutch, it cannot support the load.
Until now, applying the proposed reverse input prevention clutches to the present system needs another installation of the bearing to support the load, which means the composition of the two or three tier roller is needed. That is, the product needs more parts to support the load which cause the increasement of the cost of production, with the number of processes to assemble increased. Therefore not only the productivity is degraded, but also the size becomes large.